1. Field of the Invention
The present invention relates to method and apparatus for noise canceling and noise reducing by attenuating unwanted ambient noise from reaching the eardrum and canceling background acoustic noise received from a boom microphone or directional microphone, when used with a headset or boom headset or the like.
The invention further relates to an active noise reduction system for use in headsets, particularly in the earphone vicinity where the system utilizes a sensor microphone to detect unwanted, background noise. This noise signal outputted by the sensor microphones is processed by electro-acoustical means to produce an inverted signal so that a quiet zone is created in an acoustical waveguide located between the output transducer, and the eardrum. Therefore the desired original audio signal is not disturbed by noise when transmitted to the ear of the user. The acoustical waveguide absorbs any sound returning to the microphone from the ear (preventing feedback) and deadens any sound returning from the microphone to the ear.
This invention also relates to a noise cancellation apparatus, for use with a telephone handset or a boom microphone or directional microphones or the like, where the system utilizes two microphones, a first microphone for receiving sound comprised of speech and background noise, and a second microphone for receiving sound comprised of substantially background noise, with the means for subtracting the second signal from the first signal.
The microphone in the noise cancellation system of the present invention utilizes a two terminal system, in which the output audio signal comprised of speech and the power support input used to drive the system are transmitted on one terminal and the second terminal is grounded.
The noise cancellation apparatus of the present invention also relates to a directional microphone used in a far-field microphone device having the ability to accept acoustical sounds in certain directions better than in other directions.
The noise cancellation and noise reduction system of the present invention may be enhanced by the inclusion of an automatic audio microphone transmission feature, a sidetone feature to transmit a portion of the signal to the earcup of the speaker, and a feature to convert an active noise cancellation microphone to a standard omni-directional microphone by removing voice microphone from the circuit, and the increasing the gain of the noise microphone amplifier. This enhancement allows all audio from external surroundings to be transmitted to the earcup of the speaker by increasing the sidetone channel gain without the addition of any other microphone elements.
2. Description of the Prior Art
As is to be appreciated, in numerous situations, the presence of background acoustic noise is undesirable. As an example, consider the situation in which an operator is attempting to conduct a telephone conversation from a telephone or such similar device located in a noisy area. In this situation, loud acoustic background noise is received by a microphone in the handset of the telephone and converted to an electrical signal which is supplied to the telephone(s) of the person(s) having the conversation with the operator and is converted thereat to an acoustic signal. As a result, the person to whom the operator is communicating constantly hears the loud background noise. Further, when the person is speaking, such speech is combined with the background noise and, as such, may be difficult for the other person(s) to understand. As a result, the operator may have to shout into the microphone of the telephone. Furthermore, the signal representing the background noise is also supplied from the microphone in the operator's handset to the speaker in the operator's handset as sidetone. Thus, the operator also constantly hears the background noise from the speaker in the operator's handset and, when the other person is speaking, may impair the understanding thereof.
As another example, consider the situation in which a pilot who is operating a helicopter or the like wishes to communicate with another person by way of radio frequency (RF) communication. In this situation, the pilot typically speaks into a so-called boom microphone or boom headset which is coupled to a radio transmitting/receiving device whereupon the speech is converted into RF signals which are transmitted to a second receiving/transmitting device and converted therein to speech so as to be heard by the other person(s). As with the above situation of a telephone located in a noisy area, the loud background noise from the helicopter is received and converted into an electrical signal by the boom microphone or headset device and thereafter supplied to the receiving device. As a result, the person(s) communicating with the pilot hears the loud background noise. This may be particularly annoying when the pilot leaves the radio transmitting/receiving device in the "ON", (the hot mike) position while operating the helicopter.
As yet another example, consider voice verification and/or recognition systems into which an operator must speak for access, for instance to a physical facility or, to operate a computer or automatic teller machine. Background noise can prevent access (no recognition or verification due to background noise) or can provide false access by false verification.
In an attempt to reduce background noise so as to improve performance of a telephone or a boom microphone or headset or the like located in a noisy environment or the like, pressure gradient microphones may be utilized. Basically, a pressure gradient microphone responds to the difference in pressure at two closely spaced points. When used in an environment where the pressure gradient of the background noise is isotropic, the electrical signal produced by the pressure-gradient microphone due to such background noise is effectively zero. However, in most actual situations, the pressure gradient of the background noise is not isotropic and, as a result, in these situations, the performance of the pressure-gradient microphone is adversely affected. Additionally, since voice or speech propagates in more than one direction, the electrical signal produced by the microphone which corresponds thereto is often degraded. Thus, even if a pressure gradient microphone is utilized in either a telephone handset or a boom microphone, the desired amount of background noise cancellation may not be sufficient and the performance may not be adequate.
Furthermore, since two opposite sides of a pressure-gradient microphone respond to acoustic pressure, as previously mentioned, the handset of an existing telephone would have to be substantially modified so as to enable these two sides of the microphone to respond to the acoustic pressure. Moreover, as a result of using such a microphone in a telephone handset, the electrical signals produced therefrom should be amplified. Thus, to replace the conventional microphone in a telephone handset of an existing telephone with a pressure-gradient microphone would typically necessitate replacing the handset with a new handset and, as such, would be relatively expensive.
As an alternative to using pressure-gradient microphones, an acoustic feed-back type system may be utilized. Such a system normally includes compensation filters which are used to equalize the transfer function of the output transducers. Since the characteristics of the speakers are tightly controlled by these filters, the cost of the filters is relatively high. As a result, such acoustic feed-back systems are typically relatively expensive.
Many microphones used with noise cancellation and noise reduction apparatus are inherently nondirectional or omnidirectional, such as the electrostatic, piezoelectric, magnetic and carbon microphones. With omnidirectional small microphones, at low frequencies there is sufficient diffraction of sound around the microphone so that diaphragm motion is insensitive to the direction of the sound. At high frequencies, and correspondingly shorter wavelengths, the microphone becomes acoustically larger and shows a preference for sound arriving perpendicular to the diaphragm. Thus, the smaller in size of the microphone, the higher in frequency its behavior remains omnidirectional. Hence, the omnidirectional microphones are small compared to the wavelength and the microphone case shields the rear side of the diaphragm from receiving certain sound waves at different angles. As a result, these prior art microphones are referred to as pressure microphones since pressure is a scaler, and not a vector quantity. Thus, a directional microphone response able to increase the sensitivity of sound in a far-field region from a variety of directions is desired for a microphone device in an active noise cancellation system. That is, to achieve a directional microphone response by adding the outputs of the omnidirectional pattern and bidirectional or "figure-eight" pattern, and then simply adjusting the amplitude and phase of the summed output signal to produce the desired pattern. The figure-eight pattern is also known as a cosine pattern and is mathematically expressed a p=COS .theta., in polar coordinates. In directional microphones, distance is a factor. The distance factor measures how much farther away from a source a directional microphone may be used, relative to an omnidirectional pattern, and still preserve the same ratio of direct to reverberant pickup. Thus, the prior art has failed to provide a directional microphone in an active noise reduction apparatus based on the omni-directional patterns and the cardioid patterns where the sound pressures arriving at a determined point are added vectorially.
In devising the circuitry for an active noise cancellation apparatus for use with a boom microphone device or a directional microphone device comprising at least two microphones, it is known to use a three terminal microphone configuration. That is, a noise cancellation system having two or more microphones connected to an amplifier, for example, requires circuitry having three terminals: a power supply input terminal, an audio signal output terminal, and a ground terminal. In an effort to reduce the complexity and cost of the noise cancellation system utilized in the microphone, or boom microphone or the like which optionally may be used with a headset of the noise reduction apparatus, a two terminal microphone configuration is desired. It is desired to have a microphone configuration where the DC voltage supplied from a power supply is inputted on the same terminal as the AC audio signal outputted from the microphones, whereby the AC signal is superimposed on the DC signal. Thus, the prior art has failed to provide a two terminal microphone configuration for use in an active noise cancellation apparatus, where the power and signal are superimposed on the first terminal and the second terminal is grounded
In yet a further attempt to reduce background noise so as to improve the intelligibility of electro-acoustic communication using headsets with a microphone, a technique has been developed, called active noise reduction that utilizes a sensor microphone placed between the speaker and the ear in the sound field of the speaker, and which senses the background noise and programs audio. With this active type headphone device, a negative feedback loop is used whereby the electrical signals converted from the external noises by a microphone unit are fed back in a reverse phase for reducing the noise in the vicinity of the headphone unit. A feedback circuit utilizing a closed loop system as shown in the prior art provides a "quiet zone" between the speaker and the ear which eliminates the background noise. This is because in a noisy environment, the ear will detect not only the output of the speaker, but also the background noise.
Reference is made to the following documents providing a closed loop active noise reduction system, which documents are hereby incorporated by reference:
U.S. Pat. No. 2,972,018 to Hawley et al.
U.S. Pat. No. 3,098,121 to Wadsworth
U.S. Pat. No. 4,833,719 to Carme et al.
U.S. Pat. No. 5,138,664 to Kimura et al.
Japanese Patent Abstract No. 3-169199 to Saeki.
The above-referenced patents illustrate a variety of noise canceling devices. For instance, Hawley et al. relates to a noise reduction system for earphones having a plastic casing located between the speaker and the microphone; Wadsworth provides an earphone having a microphone located on top of the headband; Carme et al. is directed to an earphone having a hollow annular part located between the speaker and the microphone; Kimura et al. calls for a noise reduction headphone having a cup member located between a speaker and a microphone; and Saeki relates to a noise canceling headphone having a microphone located between two oppositely facing loudspeakers.
However, there exist various disadvantages in the conventional active noise reduction systems. The prior active noise cancellation systems, for instance, utilize closed loop-type circuits governed by the associated equations: ##EQU1## where P=output
S=standard audio signal PA1 H.sub.1 =high pass filter PA1 H.sub.2 =speaker at headset PA1 N=noise component PA1 B=variable gain/phase control
The conventional closed loop noise reduction system is not ideal as a very large direct transmission gain (1+BH1H2) is required in order to reduce the noise component (N) to zero at the output (P). This system suffers from the problem of instability. This creates drawback of oscillation, i.e., squealing due to the unstable loop conditions caused by variations in the transfer function of the speaker, feedback microphone and acoustic cavity containing these elements and user headgear. The degree of noise cancellation generated by the conventional closed loop noise reduction device, at any frequency, is directly related to the direct transmission gain at that frequency. However, the higher the gain the more susceptible the device is to instability.
The conventional active noise reducing headphone device also has the drawback that when mechanical vibrations such as impact, frictional induced vibrations from connecting cords, user jaw movement induced vibrations etc., are transmitted to the noise feedback microphone, these vibrational noises are converted to electrical signals by the microphone. These signals are amplified and cause instability and other non-linear effects, for example, audio interruption, loud noises or pressure surges.
Another drawback of conventional active noise reducing headphone devices is the complexity added to the device to avoid canceling the desired audio signal, which signal is inputted as an electrical signal. The desired audio signal (S) of the conventional device is input into two summing nodes to create the signal transmitted to the user's ear. The first summing node adds the negative feedback microphone signal to the desired input audio signal. But, in a conventional closed loop feedback device, the signal feedback from the microphone contains the desired audio signal as well as the ambient noise signal which is desired to be canceled. This feedback signal is subtracted from the desired input audio signal to create the anti-noise signal, with zero desired audio signal content. Then, a second summing node is used to add the desired audio signal back into the loop so it can be transmitted to the output transducer. This method 5 of generating the desired audio signal adds complexity and cost to the conventional noise reducing device. The additional summing node processing in the conventional device also increases chances of creating distortion in the desired audio signal as well as increasing the possibility of instability.
In addition, various other prior art headphone configurations have been developed for creating an active noise reduction device, where the input and output transducers are positioned in relation to the ear, such as the following three documents, which are incorporated by reference:
U.S. Pat. No. 5,134,659 to Moseley.
U.S. Pat. No. 5,117,461 to Moseley.
U.S. Pat. No. 5,001,763 to Moseley.
Moseley ('659) relates to a noise canceling system for headphones having a baffle, two speakers, and two microphones wherein the baffle serves to impede noise from traveling directly from a noise source to the input transducer by forcing the noise to travel a longer distance around the baffle and through a foam barrier. Moseley ('461) is directed to an electroacoustic function including noise cancellation for use with headbands having a microphone mounted on the headband to face in same direction of the ear canal. Moseley ('763) relates to a noise cancellation system for headbands having a speaker, microphone, and a baffle.
Thus, in general, the Moseley patents are concerned with the location of the speaker, being the output transducer, and the microphone, which is input transducer. In fact, the patents require that the speaker and microphone be in the same plane or substantially aligned in the same plane. Also, the patents teach that the processed signal output is substantially in the same time domain as the original acoustic wave, that is the signal is in phase.
In contrast to the Moseley patents, the present invention is not per se concerned with the alignment of the speaker and microphone in the same plane (although such alignment need not be explicitly excluded). The output transducer and microphone utilized in the open loop active noise reduction of the present invention may be perpendicular, tangential, or in any other location out of the same plane (as well as in the same plane). The present invention provides a noise reduction system having the capability to transmit the original input audio signal to the speaker without the readdition of the input audio signal. This is because the sensor microphone, which is the control action of the open loop, is so disposed from the audio signal, that the audio signal is not detected by the pickup or sensor microphone. That is, in the open loop system of the present invention, the original desired audio signal is transmitted to the speaker independent of the ambient noise detected by the microphone. In addition, in the present invention an acoustical material can be located between the output transducer and the eardrum of the user to create an acoustical waveguide for the transducer by coupling the audio signal to the ear of the user. The acoustical material located between the output transducer and microphone acts as an acoustic filter to decrease the open loop gain by placing an acoustical impediment in the path of the pickup microphone and the output transducer. The acoustical material isolates the desired original inputted audio signal from the noise detected and canceled by the pickup microphone. The background noise signal detected by the pick-up microphone is inverted through electric-acoustical processing means producing an anti-noise signal, which signal is transmitted to the acoustical waveguide to create a quiet zone. This quiet zone is located between the output transducer and the eardrum of the user.
Thus, the prior art has failed to provide a relatively low-cost means for reducing background noise to an acceptable level for use with communication systems or the like, and a cost-effective means for enabling existing audio communication systems to reduce background noise to an acceptable level.